Carotenoids are natural pigments which occur abundantly in the plant and animal kingdoms and which in some cases have also been produced by synthetic means. Further synthetically produced carotenoids do not appear to occur in nature. Many important carotenoids are employed as pigments in the food and feedstuff industries, e.g. for colouring egg yolk, poultry, fish and crustacea, notably in the cases of ethyl .beta.-apo-8'-carotenoate, citranaxanthin, canthaxanthin and astaxanthin. For this purpose the carotenoid pigments are added to the animals' rations as a method of imparting an enhanced and aesthetically more acceptable visual impression of colour, be it in the animal integuments, such as the skin, shanks and beaks of poultry and the skin, scales and shells of fish and crustacea, as appropriate, subcutaneous fat of poultry and the meat of fish and crustacea, or in such animal products as eggs (yolk). The enhancement of pigmentation depends on the particular light-absorbing conjugated double bond system of the carotenoid concerned, the degree of ease with which the carotenoid is taken up into the animal body following consumption of the carotenoid-enriched feed (deposition rate) and the concentration of the carotenoid or any metabolites in the target animal body tissue or product, amongst many other factors. However, from a knowledge of the structure of the selected carotenoid it cannot be predicted how effectively it functions as a pigment in this area of application. A further factor involved is the stability of the carotenoid, e.g. towards atmospheric oxidation, light, temperature and dampness, in an animal feedstuff when stored under the normal conditions to which such a feedstuff is subjected.
With respect to poultry an acceptable level and quality of pigmentation is desired for the integuments of the birds destined for consumption and for egg yolk, The use of materials to enhance yolk colour, for example, is generally promoted because consumers prefer deeply (particularly rich golden yellow) pigmented yolks. The visual appearance is indeed an important factor in the assessment of quality. Broilers and ornamental birds, for example, are in many parts of the world more aesthetically acceptable if their integuments, particularly skin, shanks and beaks, and in the case of broilers, also subcutaneous fat satisfy certain criteria of pigmentation. The need for supplementary pigmentation is especially prevalent today in view of the reduction of grass consumption with the modern methods of intensive poultry rearing, which involve the use of low fibre, high energy feeds, rendering difficult the production of well-pigmented poultry and egg yolks.
The pigmentation of fish meat and integuments, especially of various species of trout and salmon, and the meat and integuments of crustacea, especially of crabs, lobsters and shrimps, is also well known to be achieved by feeding the fish and crustacea with carotenoid-enriched feed preparations with a view to rendering the edible products more attractive to consumers.
It has now been found that certain carotenoids are surprisingly more effective as pigments in the above-indicated applications than the carotenoids known to have been used hitherto for such purposes. These carotenoids are 2'-dehydroplectaniaxanthin, desmethylspheroidenone, and spheroidenone (known), and 1-hydroxy-3,4-didehydro-1,2-dihydro-.psi.,.psi.-caroten-2-one, 5'-hydroxy-5',6'-dihydro-4'-apo-.beta.-caroten-6'-one, 1'-hydroxy-3',4'-didehydro-1',2'-dihydro-.beta.,.psi.-carotene-4,2'-dione and 5'-hydroxy-5',6'-dihydro-4'-apo-.beta.-carotene-4,6'-dione (novel). None of the known literature (including patent) references to the three known carotenoids indicate that these carotenoids can be used as pigments in the applications indicated hereinbefore, e.g. for enhancing the colour of egg yolk.